Re: [Xen-devel] PML (Page Modification Logging) design for Xen

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Kai Huang [mailto:kai.huang@xxxxxxxxxxxxxxx]
> Sent: Thursday, February 12, 2015 2:57 PM
> 
> On 02/12/2015 02:54 PM, Tian, Kevin wrote:
> >> From: Kai Huang [mailto:kai.huang@xxxxxxxxxxxxxxx]
> >> Sent: Thursday, February 12, 2015 10:39 AM
> >>>> PML needs to be enabled (allocate PML buffer, initialize PML index,
> >>>> PML base address, turn PML on VMCS, etc) for all vcpus of the domain,
> >>>> as PML buffer and PML index are per-vcpu, but EPT table may be shared
> >>>> by vcpus. Enabling PML on partial vcpus of the domain won't work. Also
> >>>> PML will only be enabled for the domain when it is switched to dirty
> >>>> logging mode, and it will be disabled when domain is switched back to
> >>>> normal mode. As looks vcpu number won't be changed dynamically
> during
> >>>> guest is running (correct me if I am wrong here), so we don't have to
> >>>> consider enabling PML for new created vcpu when guest is in dirty
> >>>> logging mode.
> >>> There are exactly d->max_vcpus worth of struct vcpus (and therefore
> >>> VMCSes) for a domain after creation, and will exist for the lifetime of
> >>> the domain.  There is no dynamic adjustment of numbers of vcpus during
> >>> runtime.
> >> Good to know.
> > could we at least detect and warn vcpu changes when PML is enabled?
> > dirty logging happens out of guest's knowledge and there could be the
> > case where user right online/offline a vcpu within that window.
> Why is the warning necessary? There's no harm leaving PML enabled when
> vcpu becomes offline.

what about online? you need enable PML for newly-online vcpu since
meaningful work may be scheduled to it within logdirty window.

> 
> Also we will not disable PML for that vcpu when it becomes offline, in
> which case we don't need to re-enable PML, which can fail, when vcpu
> becomes online again. It simplifies the logic.

offline is not a problem

Thanks
Kevin


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